The onset of ferromagnetism was studied by temperature dependence of remanent magnetization Mr. A plot of Mr vs temperature T for film thicknesses from 4.0 ML to 30 MLs is shown in Fig.1. The remanent magnetization Mr is normalized with respect to its value at 10 K, i.e., M0r . These measurements were performed while heating the sample from 10 K to 300 K. Before the measurement, the samples were magnetized along the [332] direction up to fields large enough to ensure a complete orientation of the magnetic moments. Since all these measurements were made with the magnetic field applied parallel to the easy axis. The remanent magnetization is essentially the same as the saturation magnetization, Ms. For samples above 10 ML thickness, the RT remanence changes by less than 10 % of its value at 10 K, indicating a high Curie temperature of these samples. Thinner samples below 10 ML show a drastic change in remanence with temperature. The Curie temperature decreases strongly when decreasing the thickness and reaches 120 K for the 4 ML sample which is well known in ultrathin films due to the reduced co-ordination. For the 3.5 ML Fe sample we did not observe any ferromagnetism even at 2 K. This implies that the first signature of ferromagnetic order in our films starts between 3.5 to 4.0 ML. This is approximately the same thickness for which we observe the first RHEED pattern from the Fe layer. The onset of ferromagnetism in these layers can be ascribed to a percolation phenomenon during coalescence of Fe islands similar to that observed in Fe on GaAs(001).

More details

P. K. Muduli, J. Herfort, H.-P. Sch�nherr, and K. H. Ploog, Evolution of magnetic anisotropy and spin-reorientation transition in Fe films grown on GaAs(113)A substrates by molecular beam epitaxy, J. Appl. Phys. 97 123904 (2005) (7 pages)